In the related art, a copper alloy such as Cu—Zn is used for various uses such as piping materials, construction materials, electric and electronic apparatuses, daily necessaries, and mechanical parts. In addition, a white (silver-white) tone having color fastness is required for use in decorative and construction metal fittings such as railings and door knobs, western tableware, and keys. In order to satisfy such requirement, copper alloy products may be subjected to a plating treatment such as nickel-chrome plating.
However, such plated products have a problem that a plating layer on a surface thereof is peeled off after long-term use. In addition, the bactericidal and antibacterial properties of the copper alloy deteriorate. Therefore, a glossy white Cu—Ni—Zn alloy is proposed.
As such a Cu—Ni—Zn alloy, for example, JIS C 7941 specifies a free-cutting nickel silver containing Cu (60.0 mass % to 64.0 mass %), Ni (16.5 mass % to 19.5 mass %), Pb (0.8 mass % to 1.8 mass %), Zn (balance), and the like. In addition, Patent Document 1 discloses a white copper alloy containing Cu (41.0 mass % to 44.0 mass %), Ni (10.1 mass % to 14.0 mass %), Pb (0.5 mass % to 3.0 mass %), and Zn (balance). In addition, Patent Document 2 discloses a lead-free white copper alloy containing Cu (40.0 mass % to 45.0 mass %), Ni (5.0 mass % to 20.0 mass %), Mn (1.0 mass % to 10.0 mass %), Bi (0.5 mass % to 3.0 mass %), Sn (2.0 mass % to 6.0 mass %), and P and Sb (at least one kind; 0.02 mass % to 0.2 mass %).
However, since the copper alloys disclosed in JIS C 7941 and Patent Document 1 contain a large amount of Ni and Pb, there are problems pertaining to health and sanitation and thus, the use thereof is limited. Ni causes Ni allergy which is particularly severe among metal allergies, and Pb is a well-known harmful material. Therefore, there are problems for use in construction metal fittings such as railings, which may be touched by human skin, and daily necessities such as home appliances. In addition, when a large amount of Ni is contained, workability such as hot workability and press property deteriorates and, because Ni is expensive, the production cost is increased. Therefore, the use thereof is limited.
In addition, the copper alloy disclosed in Patent Document 2 does not contain Pb, which is harmful to human body, and improves workability (machinability) using Bi. However, Bi is a metal having a low melting point. Therefore, in the case of a copper alloy, since Bi is present in a matrix as the metal substantially without being dissolved therein, Bi is melted during hot rolling and there is a problem in hot workability. In addition, Ni, Sn, and Bi are expensive metals, and thus there are problems in terms of cost and production when these metals are added in large amounts.
In addition, in a Cu—Zn—Ni alloy plate disclosed in JIS H3110 (plates and strips of phosphor bronze and nickel silver) of the related art, 8.5 mass % or greater of Ni and 60 mass % or greater of Cu are contained; or the Zn concentration is less than 30 mass %. Since a metal structure of such a plate is the α single-phase structure at a high temperature and normal temperature, hot workability is low. Therefore, such a Cu—Zn—Ni alloy is produced by casting an ingot with a cross-section having, for example, a thickness of about 15 mm and a width of about 400 mm without hot rolling; heating the ingot at a high temperature of about 700° C. for several hours; performing a homogenizing heat treatment for alleviating the segregation of components during casting; and repeating cold rolling and annealing. The productivity thereof is lower than that of an ingot for hot rolling with a cross-section having, for example, a thickness of about 200 mm and a width of about 800 mm. In addition, even when the homogenizing heat treatment is performed at a high temperature for a long period of time, the degree of segregation of alloy components is larger than that of a hot-rolled plate subjected to hot rolling, which causes a problem in quality. In particular, in a plate having, for example, a thickness of 1 mm or higher obtained through production processes in which an annealing process is performed only once or twice; a plate obtained through production processes in which an annealing process is performed multiple times and a retention time after heating at a recrystallization temperature or higher is shorter than 30 minutes; and a plate obtained through production processes in which an annealing time is long and an annealing temperature is lower than (recrystallization temperature+100° C.), the segregation is not eliminated.
In addition, a copper alloy is known to have bactericidal action. In medical institutions such as hospitals, there are cases in which a patient is infected with bacteria having resistance to drugs such as antibiotics, for example, staphylococcus aureus or pseudomonas aeruginosa (generally called hospital infection), which is a serious problem. There are many infection routes of bacteria through hospital infection. For example, bacteria spread by a patient with bacteria touching an object; and another patient or medical staff touching that touched object. It is expected that, by forming an object, which may be touched by patients or medical staffs, from a copper alloy, these bacteria are destroyed or reduced, infection routes are removed correspondingly, and thus hospital infection is reduced. For example, it is expected that, by forming knobs, lever handles, door handles, and the like, which are inside the hospital, from a copper alloy, infection routes of bacteria are reduced. In addition to the case of hospital infection, infections with various bacteria in public places such as trains, buses, or parks can be prevented by forming a component, which may be touched by an unspecified number of people, from a copper alloy having bactericidal and antibacterial properties.
However, when these knobs, lever handles, door handles, and the like formed from a copper alloy are used in practice, there is a difference in color tone between a portion, which are often touched by human body, and the other portions. In addition, after long-term use, since a discolored layer (oxide) is slowly formed in the portion which are often touched by human body or the portion is physically removed, the color tone thereof is different from that of the other portions (portions which are barely touched by human body). Accordingly, the external appearance looks bad. Therefore, since almost all the copper alloy knobs for these uses are used in a state where a copper alloy surface is coated with a plating film or a clear coating, the bactericidal and antibacterial properties of a copper alloy are not exhibited.